Filter for an aquarium



FIG. 1 is a perspective view of a filter for an aquarium showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof;

FIG. 6 is a left side view thereof; and,

FIG. 7 is a right side view thereof.

The logo shown in broken lines in the Figures is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a filter for an aquarium, as shown and described. 